Sensor devices have been proposed that include a sensor. Resistance values in the sensor change according to changes in the environment (e.g., pressure changes, air temperature changes, humidity changes, etc.). The environment change is detected based on the resistance change.
In Japanese Patent Publication No. 9-147283, for instance, an oscillating frequency of a CR oscillating circuit changes according to the sensor resistance value changes caused by environment changes (e.g., a change of density of a measured object due to moisture, etc.). A square wave pulse signal of a frequency corresponding to this oscillating frequency is outputted. The change of density is detected on the basis of a changing ratio of the oscillating frequency with respect to a fundamental oscillating frequency.
However, when the density change is small, the change of the resistance value of the sensor is small, and the changing ratio of the oscillating frequency is also small. Accordingly, it is difficult to detect a slight density change.
If an output bit number of an A/D converter is increased, the resolution of the sensor output can be improved. However, a data width of the sensor output is widened by increasing the output bit number. Accordingly, processing load of the sensor output is increased.
Further, an input voltage of the A/D converter is an electric power voltage or less. Accordingly, for example, when the electric power voltage is a low voltage (e.g., several V to ten several V) and the resolution is improved by increasing the output bit number of the A/D converter, a minimum detecting voltage unit is reduced. As a result, if noise is included in the input voltage of the A/D converter, the A/D converter will easily detect the noise.
Moreover, when a dynamic range of the detecting object is wide and the detection is to be performed by one sensor, the resolution is reduced. On the other hand, when the detecting range is borne by plural sensors, the number of parts is increased.
Therefore, as shown in Japanese Patent Publication No. 2003-273673, an analog circuit for adjusting a gain or an offset may be added to a detecting circuit of the sensor output. However, using this device, the circuit scale becomes relatively large.
Accordingly, there remains a need for a sensor device having high resolution, a wide detecting range, and a small scale.